The present invention relates to reciprocating engines, and more particularly to reciprocating engines having boundary (marginal) lubrication rather than conventional hydrodynamic (full film) lubrication systems.
Despite the major advances in engine technology over the past ten to fifteen years, oil consumption and oil deterioration still remain as two obstacles which must be overcome before further improvements can be made in today's reciprocating engine. Oil consumption and deterioration are considered the result of the presence of oil in the piston ring/cylinder liner region. Oil must be supplied to this area to provide hydrodynamic lubrication for the sliding rings and piston skirt.
In order to more fully understand the factors involved in piston ring design/lubrication theory, six major categories or cases of piston ring design and lubrication have been identified. These may be described as:
(a) Cast iron rings and liner; hydrodynamic lubrication (Case A);
(b) Ceramic rings and liner; hydrodynamic lubrication (Case B);
(c) Cast iron rings and liner; unlubricated (Case C);
(d) Ceramic rings and liner; unlubricated (Case D);
(e) Cast iron rings and liner; boundary-film lubrication (Case E); and
(f) Ceramic rings and liner; boundary-film lubrication (Case F).
The first category (Case A), cast iron rings and liner with hydrodynamic lubrication, is currently used in virtually all reciprocating engine design. This design depends on the lubricant to provide a minimum oil film thickness between the ring and liner to prevent metal-to-metal contact. The oil also serves to transfer heat from the piston rings to the cylinder wall. The primary factors which affect piston ring-cylinder wall friction with this design are the temperature and characteristics of the lubricant.
Ceramic rings and cylinder liners may also be operated with hydrodynamic lubrication (Case B). This is not common, or even practical, due to the high cost of ceramic components and the negligible difference in friction levels compared with conventional cast iron components. The insignificant difference in friction is due to the fact that there is limited interaction between the ring surface and the cylinder wall surface as a result of the presence of the oil film. Thus, the lubricant's properties determine the major friction levels with hydrodynamic lubrication regardless of the material used for the rings or liner.
Unlubricated cast iron rings and liners (Case C) will not work for the same reasons that boundary-film lubricated cast iron components will not work as will be discussed below. Unlubricated ceramic components (Case D) can be used successfully with ion-implantation to reduce the friction and wear. Such a method is disclosed in U.S. Pat. No. 4,775,548. However, this type of ceramic operation (Case D) is not involved for the purposes of this invention.
Ceramic rings and cylinder liners are considered necessary for boundary-film lubrication (Case F) because conventional cast iron rings and liners can not survive under these conditions (Case E). There are two theories which explain the successful operation of ceramics under boundary-film lubrication compared to the failure of cast iron components under identical conditions.
Under boundary-film lubrication, there is no hydrodynamic oil-film present for lubrication. Instead, only a small amount of oil is adsorbed into the surface of the two materials. Thus, the sliding surfaces are allowed to contact each other during the entire piston stroke cycle of the engine. During sliding motion, the surface asperities of the two materials come into contact with each other and tend to shear at their base. If the melting point of either material is low enough, the asperities will melt as they shear at their base causing the friction force to increase significantly. This is the case with cast iron components, and this phenomenon is commonly referred to as scuffing. Typically, scuffing leads to a runaway condition where more and more asperities come into contact with each other after the highest peaks have sheared. This leads to an increasing contact area whose asperities are being welded together and results in engine seizure.
However, the melting point of the ceramics is much higher than that of the case iron. As a result, the asperity peaks of the ceramics are not prone to melt and shear as they come into contact with each other. Thus, the friction force remains low and engine failure is not a problem.
The invention described herein provides benefits from reduced oil consumption and oil deterioration which include reduced particulate emissions, reduced operating costs, and increased oil change intervals.